WARNING: The following contains mild spoilers for Avengers #38 by Jason Aaron, Ed McGuinness, Mark Morales, Jason Keith and VC's Cory Petit, on sale now.
The machinations of Mephisto have impacted the lives of the Avengers for millennia, and the devilish villain just revealed that he's secretly played a part in numerous notable battles across the course of human history. It turns out he also had a hand in extending a massive, long-lasting conflict between two of Marvel's most powerful warriors.
Avengers #38 revealed that Thor and Apocalypse had another titanic duel centuries earlier, and it's one that involved some of the Marvel Universe's strongest forces.
As two beings who have been alive for centuries, Apocalypse and Thor have come across each other's paths many times throughout the ages. The earliest encounters between Thor and Apocalypse were revealed over the course of Rick Remender and John Cassaday's Uncanny Avengers. Roughly a thousand years before the current Marvel Universe, the Pharoah Rama-Tut, an alias for the time-traversing villain Kang the Conqueror, tricked Apocalypse into believing Thor would one day stand in the way of his ultimate victories. Finding Thor before the arrogant young god had earned the right to wield Mjolnir, their first battle resulted in a victory for Apocalypse and Thor forced to retreat.
However, after imbuing his ax, Jarnbjorn, with Celestial-dampening magics, Thor was able to defeat Apocalypse and save the direct ancestor of Wolverine. While the two would go on to battle again in the modern Marvel Universe, Avengers has now revealed that Thor battled Apocalypse again at some point centuries ago in Egypt. This time, the battle was urged on by Mephisto, who came to Apocalypse as a serpent and whispered schemes into his ears. Convincing Apocalypse that "false gods" like the Asgardians stood in his way, Apocalypse and Thor fought once again.
However, there are notable differences between this battle and the previous ones. By this point, Thor has gained the ability to use Mjolnir, as he is briefly seen utilizing it in their battle. It also looks like Thor has potential allies in this battle, including the Sorcerer Supreme of that era. The Phoenix Force is also present at the scene, hanging over the pyramids of Egypt as the battle rages. Notably, Thor doesn't seem to regard the Phoenix Force with respect or reverence. Instead, citing his experience with the Phoenix -- both in and out of combat -- Thor notes that he doesn't trust the Phoenix Force at all. This adds additional layers of conflict to the interplay between the ancient forces, how they perceive each other, and how they might factor into whatever plans Mephisto has for the rest of the world.
All of this suggests that the battle between these ancient beings might have been motivated by more than just their hatred for each other. There are even the revelations from "X of Swords" about Apocalypse, which have added a tragic edge to the mutant's life by casting him as someone who was not strong enough to fight alongside his family. With that having motivated a dedication to the idea of "survival of the fittest" within him, Apocalypse's ancient battles with Thor show that he had something to prove long before he menaced the X-Men.